This invention relates to an orifice plate construction for a glass fiber forming bushing.
In the manufacture of glass fibers, a bushing with a bottom plate having 400 to 2000 minute orifices therein is charged with molten glass, which then flows down through the orifices to draw the glass into fibers or filaments. The orifice plate typically becomes downwardly deformed in a convex manner during prolonged usage, however, owing to the weight of the molten glass in the bushing, the tension of the withdrawn fibers, and the high operating temperatures of from 1100.degree. C. to 1300.degree. C. Such deformation causes a difference in heat radiation between the cones formed in the peripheral areas of the orifice plate and those formed in the central area thereof, and in addition the peripheral cones tend to drift or migrate toward the central area of the deformed orifice plate. This reduces the stability of the cones and increases filament breakage, and when adjacent cones merge or become joined together it is difficult to restore their separation.
To overcome this difficulty the distance between the orifices or the thickness of the orifice plate may be increased, or the plate may be strengthened by reinforcing ribs. These conventional solutions are disadvantageous, however, in that when the size of the orifice plate is increased the size of the overall bushing is correspondingly increased. As a result it is difficult to uniformly heat the molten glass and its orifice flow properties therefore become uneven, which leads to filament breakage and an attendant reduction in productivity. Even when the filaments do not break their diameters vary, which reduces the strength of the fibers. In addition, as the bushings are made of platinum, any increase in their size involves corresponding increases in equipment investment and in the cost of the filaments produced.
An alternative technique has been to use a relatively smaller bushing having 4000 to 6000 orifices in the flat bottom plate at intervals of less than 3 mm, and to direct an air flow at the bottom of the orifice plate to cool the cones of molten glass and increase their viscosity, thus preventing the cones from joining together. This technique increases productivity, but suffers from the disadvantage that as the orifice density is increased the orifice plate becomes necessarily weakened, which enhances its convex deformation tendency. Such deformation or curvature results in a non-uniform cooling effect from the air flow, which causes the fibers to break off, vary in diameter, etc.
Thus, in spite of the many and varied prior art approaches, orifice plate deformation remains a significant problem in the industry.